I tried it twice (sli that is) in a couple different configurations. First setup was a pair of 6800 Ultras. It was "ok", but there weren't enough high-end games to really take advantage of it and the ROI just wasn't worth it.

My second effort was as an early adopter of the 7950GX2...big mistake. It cranked some serious frames in a few games, but required so much effin tweaking and never really got stable drivers so I ditched it for a single 8800GTS (G92).

Now I run a single GTX285 1GB card and that works fine. I'm fortunate to have a good job and the assets to direct towards the improvement of my pc/gaming hobby, but sli just doesn't have the performance in the few games I play to warrant it...right now anyway.